Without limiting the scope of the invention, its background is described in connection with the recovery of oil from subterranean oil-bearing formations. The present invention relates to post-primary recovery of oil from subterranean oil-bearing formations and includes improved techniques for enhancing the oil displacement efficiency of a post-primary oil recovery process.
Generally, water flooding and surfactant flooding are processes well known in the art to recover the vast quantities of oil which remain in the formation after primary oil recovery operations and it has been common to use surfactants and surfactant systems for oil recovery. Surfactants contain a hydrophobic part and a hydrophilic part at opposite ends of a long molecule that tend to orient at an interface with its hydrophobic portion in the oil and its hydrophilic portion in the aqueous phase. For liberating oil from a petroleum reservoir, a surfactant must, in general, stay in the interface in order to lower the interfacial tension. It must be heavy enough that normal thermal perturbations do not displace it into one phase or the other and be able to reduce the interfacial tension between oil and aqueous reservoir fluid from around 30 dynes per centimeter to a few millidynes per centimeter or less. In addition the surfactant must be able to move into and out of the surface in an unhindered manner and not function as an emulsifier in the usual sense, producing, as it does, an unstable emulsion.
Alkaline-surfactant-polymer (ASP) flooding was developed to reduce the interfacial tension between oil and water to displace the discontinuous trapped oil remaining after the waterflood. Alkaline-surfactant-polymer flooding is described in commonly owned and co-pending U.S. patent application Ser. No. 12/879,231, filed Sep. 10, 2010.
United States Patent Application No. 20080312108 (Berger and Berger, 2008) discloses compositions and process for recovering of oil from subterranean oil-bearing reservoirs consisting of green non-toxic biodegradable strong alkali metal salt of polymerized weak acids, one or more surfactants, an aqueous fluid, and optionally one or more mobility control agents and optionally one or more co-solvents. Such compositions are injected into the reservoir through one or more injection wells and assist in recovering trapped oil through one or more producing wells. The compositions and the process described in the invention offer the advantage of improved compatibility with unsoftened waters, surfactants, and various mobility control agents. The green non-toxic, biodegradable properties of the alkali makes it particularly suitable for environmentally sensitive applications such as offshore and inland lakes
U.S. Pat. No. 4,004,638 issued to Burdyn et al. (1977) teaches recovery of oil from subterranean oil reservoirs by water flooding employing an alkaline agent and a sulfonate surfactant. An aqueous initiation slug containing an alkaline agent selected from the group consisting of alkali metal and ammonium hydroxides is injected into the reservoir via a suitable injection system. Thereafter an aqueous surfactant slug is injected into the reservoir behind the initiation slug. The surfactant slug contains a sulfonate surfactant and an alkaline agent. Subsequent to injection of the surfactant slug, an aqueous flooding medium is injected in order to displace the oil within the reservoir to a production system from which it is recovered. A portion of the flooding medium may contain a thickening agent for mobility control purposes.
U.S. Pat. No. 4,976,315 issued to Prukop and Chea (1990) discloses a method for increasing the recovery of oil in enhanced oil recovery operations employing anionic surfactant by blending a taurine with said anionic surfactant. The taurine may also increase the salt and divalent ion tolerance of the anionic surfactant.
Sulfonate surfactants have been the exclusive choice for high temperature application due to presumed instability of ether sulfate (ES) surfactants. As sulfonates in general are more expensive than sulfates, the costs were prohibitively high in some cases for enhanced oil recovery (EOR) to be even considered.
U.S. Pat. No. 4,331,543 issued to Wilson and Pao (1982) describes a process for the recovery of oil from subterranean oil reservoirs by waterflooding employing ether-linked sulfonate surfactants in which oxidative degradation of the surfactant is retarded through the establishment of an anaerobic condition in the surfactant solution or through the use of oxidation inhibitors. According to the '543 patent the anaerobic condition may be provided by mechanical means such as scrubbing the injected water with an inert gas in order to remove oxygen or by employing produced well water which is handled under a closed system to exclude oxygen. A preferred class of oxidation inhibitors is sterically hindered phenolic compounds which function as free radical chain inhibitors.
U.S. Pat. No. 3,943,160 issued to Farmer et al. (1976) describes a waterflood oil recovery process, in which a mixture of petroleum sulfonate and alkoxylated alcohol sulfate surfactants is injected into a reservoir to displace oil, which is improved by using a sulfate surfactant that contains at least one chain-branching substituent on a carbon atom alpha or beta to the sulfate group. In a reservoir that is relatively hot, the improved process provides good oil-displacement efficiency and polyvalent metal ion compatibility in addition to improved stability towards hydrolytic decomposition of the sulfate surfactant.
A large number of petroleum reservoirs have some hardness (divalent cations) in the water, and thus pose a great threat to the aqueous stability of the chemicals injected. Divalent cations precipitate with alkali, surfactant and polymer and result in plugging during injection. In addition, hardness has a dominant impact on phase behavior especially at low concentration of surfactant (Nelson, 1984) and this can cause high surfactant retention by ion exchange between the surfactant, brine, and clay (Hill, et al., 1977; Hirasaki, 1981). Therefore, extra measures are taken to test the compatibility of the injection chemicals with brine. One measure is softening of injection water, which is an expensive process that in some cases is not feasible.